from Wiktionary, Creative Commons Attribution/Share-Alike License

from the GNU version of the Collaborative International Dictionary of English

adj. Firm as a stub or stump; stiff; unbending; unyielding; persistent; hence, unreasonably obstinate in will or opinion; not yielding to reason or persuasion; refractory; harsh; -- said of persons and things

from The Century Dictionary and Cyclopedia

Sturdy; stout; strong.

Fixed or set in opinion or purpose; obstinately determined; inflexibly resolute; not to be moved by persuasion; unyielding.

adj. tenaciously unwilling or marked by tenacious unwillingness to yield

Etymologies

Middle English stuborn.

(American Heritage® Dictionary of the English Language, Fourth Edition)

From Middle English stoburn, stoburne, styburne, stiborn; probable origin *stybor, *stibor, apparently from Old English styb ("a stump, stub") + adj. formative -or as in Old English bitor, English bitter. (Wiktionary)